1. Field of the Invention
This invention relates to compound microlenses and, more particularly, to compound microlenses that are controlled by a micro-electromechanical structure or subsystem (MEMS).
2. Discussion of the Related Art
In the optoelectronic art optical lenses have many applications as both individual lens elements (single or compound lenses) or as arrays of such elements. Individual lens elements are used, for example, to couple an optical source (e.g., a laser) to an optical receptor. Receptors include well-known optical waveguides (e.g., optical fibers and silica waveguides), well-known photodetectors (e.g., p-i-n and avalanche photodiodes), and other optical devices. On the other hand, an array of such lenses can perform the same coupling function between an array of optical sources and an array of optical receptors. The coupling function may include one or more of the following function species: focusing, collimating and shaping.
State-of-the-art lens arrays include microlenses that are etched in a semiconductor (e.g., Si) or a dielectric (e.g., a silica-based glass) body. There are many techniques to fabricate such a microlens array. Most involve standard photolithographic processing techniques. Whenever a master is available, a microlens array can be duplicated using molding techniques. For many applications, large focal length uniformity across all lenses is desired, but is not necessarily achieved because of defects in the materials and variations in the process (e.g., in the etch profile). One approach to solving the latter problem is described by C. Bolle in copending U.S. patent application Ser. No. 10/010,570, entitled Method for Compensating for Nonunifrom Etch Profiles. The application, which was filed on Nov. 13, 2001 and assigned to the assignee hereof, is incorporated herein by reference. Another prior art technique for making Si microlenses is described by L. Erdmann et al., Opt. Eng., Vol. 36, No. 4, pp. 1094-1098 (1977), which is also incorporated herein by reference.
In theory at least, such individual microlenses or microlens arrays can be fabricated from other materials such as plastic. In practice, however, the choice of material often depends on the precision demanded by the particular application. For example, many optoelectronic applications discussed below require extremely high precision in the way that light beams are coupled from one device/element to another. These applications dictate the use of a material (e.g., Si) that has a mature processing technology that enables the microlenses to be shaped with corresponding precision.
A microlens array is an essential component for many types of optical subsystems, such as optical switches, routers, attenuators, filters, equalizers and dispersion compensators. In typical applications, the microlens array is used to collimate optical beams from an array of fibers or lasers and to focus them onto an array of receptors.
Conventional optical routers and switches use arrays of microlenses to collimate/focus optical beams from an array of optical input fibers to an array of optical output fibers, so that the coupling between the two arrays is efficient. The collimating and focusing functions serve to match the diameter of the optical beams to the aperture of the optical fibers.
In a MEMS structure, such conventional microlens arrays generally do not provide optimal coupling of the optical beams to the optical fibers for several reasons. First, the lens curvature varies from lens to lens due to limited manufacturing tolerances. These curvature variations lead to focal length variations, which, in turn, lead to optical beam diameter variations in the optical output fibers. Second, optical path lengths between different pairs of input and output fibers vary for different routings, which leads to variations in beam diameters at the output fibers.